Periodic optical structures, such as photonic crystals, metamaterials, and the like, play an important role in optoelectronic devices. Periodic optical structures are designed to affect the motion of photons using the physical phenomenon of diffraction. This is similar to the way that periodicity in a semiconductor crystal affects the motion of electrons. Specifically, periodic optical structures are made from periodic dielectric or metallo-dielectric structures that are designed to affect the propagation of electromagnetic waves in the same way as the periodic potential in a semiconductor crystal affects the electron motion by defining allowed and forbidden electronic energy bands. For example, the absence of allowed propagating electromagnetic modes inside the photonic crystal structures, in a range of wavelengths called a photonic band gap, gives rise to distinct optical phenomena such as inhibition of spontaneous emission, high-reflecting omni-directional mirrors, and low-loss or lossless waveguiding, among others. In addition to some of the optical phenomena observed for photonic crystal devices, metamaterials that have negative refractive indices exhibit properties that include a Doppler effect reversal (i.e., a light source will appear to reduce, instead of increase, its frequency as it moves towards you) and a Snell's law reversal (i.e., light rays will refract on the same side of the normal vector upon entering the material), among others.
Owing to this ability to control and manipulate the flow of light, periodic optical structures find use in many applications. For example, two-dimensional photonic crystals, in the form of thin-films, are found in low- and high-reflection coatings on lenses and mirrors as well as in color-changing paints and inks; and metamaterials have been used to produce superlenses, or lenses that exceed the diffraction limit. In addition, based on the potential to offer lossless control of light propagation at a size scale near the order of the wavelength of light for photonic crystal devices, and at a size scale smaller than the wavelength of light for metamaterials, periodic optical structures have the potential to produce the first truly dense integrated photonic circuits and systems (DIPCS). Individual components that are actively being developed include resonators, antennas, sensors, multiplexers, filters, couplers, switches, superlenses, frequency-doubling devices, parametric amplifiers, parametric oscillators, and the like. The integration of these components would produce DIPCS that would perform functions such as image acquisition, target recognition, image processing, optical interconnections, Analog-to-Digital conversion, sensing, and the like. Further, the resulting DIPCS would be very compact in size and highly field-portable.
Since the basic physical phenomenon is based on diffraction, the periodicity of the periodic optical structure has to be in the same or smaller length-scale as approximately half the wavelength of the electromagnetic waves. For example, applications using light at telecommunications wavelengths require structures to be fabricated at microscale and nanoscale dimensions. This, however, is the major challenge to commercial implementation of periodic optical structure-based devices. More specifically, there is currently a lack of systematic fabrication procedures for reliable and reproducible production of microscale and nanoscale periodic optical structures with sufficient precision to prevent scattering losses blurring the optical properties.
Most techniques for fabricating 2- and 3-dimensional periodic optical structures are based on those used for integrated circuits, such as photolithography and etching techniques. However, these methods are not optimal. To circumvent these methods, which require complex machinery, alternative approaches have been proposed. These include, for example, self-assembling photonic crystal structures from colloids or using fiber drawing techniques developed for communications fiber to grow photonic crystal-fibers. However, these methods do not lend themselves well to reliable and reproducible commercial scale production.
Accordingly, there remains a need for improved methods of producing periodic optical structures or devices. It is to the provision of such methods that the various embodiments of the present invention are directed. More specifically, it is to the provision of improved photo-masks for use in fabricating periodic optical structures, as well as the associated periodic optical structures fabricated therefrom, that the various embodiments of the present invention are directed.